TECHNICAL FIELD OF THE INVENTION

The present invention relates to dielectric waveguides for millimeter-wave communication links and, more particularly, to dielectric waveguides having dielectric properties that add low transmission loss to communication links operating at millimeter-wave frequencies.

BACKGROUND OF THE INVENTION

Communication links based on optical fibers have been increasingly employed as an alternative to full electrical-wire links for high-speed data transmission, for instance at transmission signal wavelengths in the range from 200 nm to 2000 nm. In view of the constant need for increased (data rate) bandwidth, electrical-wire link length is limited by transmission losses whereas optical links are feasible and widely used for long link lengths. On the other hand, conventional optical link solutions become less interesting with decreasing link length due to the costs associated with non-silicon semiconductor technology, fiber alignment and surface precision.

Dielectric waveguides/fibers have proven to be useful for transmitting millimeter-wave signals with low losses and are promising candidates for new communication systems which use carrier frequencies around 85 GHz and 140 GHz. However, since the transmitting power of integrated circuits operating within the high frequency range is limited and the respective receivers require a minimum signal input power level, waveguides with very low attenuation levels are required in order to obtain sufficient link length. In dielectric waveguides consisting of a pure dielectric material and containing no metal parts, the electromagnetic field distribution of the traveling wave subdivides into a part that propagates within the dielectric core and a part that propagates within the medium surrounding the fiber, which is usually air. However, due to the dielectric loss tangent, tan δ, of most available dielectric materials, such dielectric waveguides add too high transmission losses to the communication link. Research studies appear to show that conventional dielectric waveguides having very low transmission losses guide the propagating field mainly in the surrounding air. For this reason, in order to obtain lower losses along the length of the fiber, conventional dielectric fibers are usually configured such as to ensure that a significant part of the electromagnetic field propagates in the medium surrounding the fiber. However, such conventional configurations have the drawback that integrity of the transmitted signal becomes degraded, or may even be interrupted, due to the interaction between the electromagnetic field propagating in the medium surrounding the fiber and external agents, such as a finger or hand that touches the dielectric waveguide. For instance, it has been shown that a 12.7 Gbps link at 120 GHz over 1 meter can be realized with dielectric waveguides based on conventional polymer materials. However, the signal transmission along the fiber can be interrupted if a user touches the fiber.

Patent application publication No. US 2014/0368301 A1 describes the addition of a metallic shield around the core and cladding layers of dielectric fibers as a means for reducing electromagnetic interference between the transmitted signal and external objects. However, dielectric waveguides including metallic shields, such as cooper or aluminum shields, are rather costly. Moreover, the manufacturing process of waveguides integrating polymers and metallic materials is more elaborate and time consuming than of waveguides exclusively made of polymer materials. In addition, since available dielectric waveguides are configured so that most of the propagating electromagnetic field travels within the medium (air/foam) surrounding the dielectric core, such waveguides present a rather large cross-section that may pose limitations to their use in applications requiring a compact design.

Hence, there is still a need for dielectric waveguide solutions for transmission of millimeter-wave signals that add low transmission loss to the communication links and capable of preserving signal integrity along the dielectric waveguide, and which can be produced in a simple and cost- effective manner while having a smaller diameter than the existing prior art.

SUMMARY OF THE INVENTION

The present invention has been made in view of the shortcomings and disadvantages of the prior art, and an object thereof is to provide a dielectric waveguide for millimeter- wave signal transmission that adds low transmission loss and preserves signal integrity along the communication link, while presenting lower production costs than waveguide configurations with metallic parts, as well as a low-loss cable comprising the same.

This object is solved by the subject matter of the appended independent claims. Advantageous embodiments of the present invention are the subject matter of the appended dependent claims.

The present invention provides a dielectric waveguide for transmission of millimeter-wave signals, comprising: an inner core comprising a dielectric medium adapted to transmit a millimeter-wave signal by carrying an electromagnetic field along the dielectric waveguide; wherein the dielectric medium comprises a dielectric material having dielectric properties adapted to confine the propagating electromagnetic field to the inner core while adding low transmission loss at signal frequencies in a millimeter- wave frequency range. In a further development, said dielectric medium is characterized by a dielectric constant higher than 3.0 and a loss tangent lower than 0.001 at signal frequencies in said millimeter-wave frequency range.

In a further development, said millimeter-wave frequency range includes frequencies in the range from 50 GHz to 300 GHz. In a further development, the dielectric waveguide further comprises a cladding surrounding the inner core, the cladding being adapted to further confine the electromagnetic field to the inner core at frequencies in said millimeter-wave frequency range.

In a further development, said cladding is directly adjacent to said inner core and is made of a non-metallic material. In a further development, said cladding layer is made of a polymer material with a loss tangent lower than 0.001 at frequencies in said millimeter-wave frequency range.

In a further development, said dielectric material includes quartz with a dielectric constant higher than 3.0 and a loss tangent lower than 0.0001 and/or alumina.

According to a further development, said dielectric medium is a solid of said dielectric material. According to a further development, said dielectric medium comprises a powder and/or granulate of said dielectric material that fills a volume of the inner core.

According to a further development, said dielectric medium comprises one or more bundles of a plurality of fibers made of said dielectric material.

According to a further development, each of the plurality of fibers of at least one bundle extends along the inner core.

According to a further development, the plurality of fibers of at least one bundle forms a yarn that extends along the inner core, each fiber in the yarn having a length shorter than the length of the inner core.

According to a further development, less than 3% of the propagating electromagnetic field is carried by a medium surrounding the dielectric waveguide at signal frequencies in said millimeter-wave frequency range.

The surrounding medium may be air. The present invention also provides a cable for transmitting signals at millimeter- wave frequencies, comprising the dielectric waveguide of the present invention.

The accompanying drawings are incorporated into the specification and form a part of the specification to illustrate several embodiments of the present invention. These drawings, together with the description serve to explain the principles of the invention. The drawings are merely for the purpose of illustrating the preferred and alternative examples of how the invention can be made and used, and are not to be construed as limiting the invention to only the illustrated and described embodiments. Furthermore, several aspects of the embodiments may form, individually or in different combinations, solutions according to the present invention. The following described embodiments thus can be considered either alone or in an arbitrary combination thereof.

BRIEF DESCRIPTION OF THE FIGURES

Further features and advantages will become apparent from the following more particular description of the various embodiments of the invention, as illustrated in the accompanying drawings, in which like references refer to like elements, and wherein: Fig. 1 is a schematic view of a dielectric waveguide according to an embodiment of the present invention;

Fig. 2 is a schematic view of a dielectric waveguide according to a further embodiment of the present invention; and

Fig. 3 is a schematic view of a dielectric waveguide according to a further embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be more fully described with reference to the figures, in which exemplary embodiments of the invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that the disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

A dielectric waveguide 100 for transmission of millimeter-wave signals will now be described with reference to Fig. 1 . The dielectric waveguide 100 comprises an inner core 1 10 that extends along the dielectric waveguide length L, and comprises a dielectric medium 120 for transmitting the millimeter-wave signal along the dielectric waveguide 100 by carrying an electromagnetic field associated with the millimeter-wave signal. The dielectric medium 120 comprises, or is made of, a dielectric material having dielectric properties that are selected so as to confine the propagating electromagnetic field to the inner core 1 10 while introducing low transmission loss at signal frequencies in the millimeter-wave frequency range, such as for instance, in the frequency range of 50 GHz to 300 GHz.

In order to achieve low transmission loss and low attenuation properties, the dielectric material of the dielectric medium 120 is selected so as to exhibit a dielectric constant ε _Γ higher than 3.0 and a loss tangent, tan δ, lower than 0.001 at signal frequencies in the millimeter- wave frequency range. The use of a dielectric material with a high dielectric constant value (ε _Γ > 3.0) for the dielectric medium 120 leads to a significant portion of the electromagnetic field being confined within the dielectric medium 120 while only a reduced portion of the field propagates in the medium surrounding the inner core 1 10. As result, the signal propagation along the dielectric waveguide 100 is relatively insensitive to touch or electromagnetic coupling with surrounding objects, while adding low transmission loss along the waveguide 100. For instance, the dielectric material chosen for the dielectric medium 120 may be quartz, which may exhibit a dielectric constant ε _Γ higher than 4.0 and a loss tangent tan δ lower than 0.0001 at millimeter-wave frequencies. Published values of the dielectric constant of quartz vary between 3.5 and 4.3 within the considered frequency range of 50 GHz to 300 GHz, dependent of the material purity. Another example of a low loss, low attenuation dielectric material that can be used for the dielectric medium 120 of the inner core 1 10 is alumina. Alumina may exhibit a high dielectric constant ε _Γ of about 9.0 and a low loss tangent, tan δ < 0.0007, (dependent on the degree of purity) at frequencies around 100 GHz.

Although the dielectric properties of the dielectric medium 120 may be sufficient for providing the desired low transmission loss and low sensitivity to the surrounding environment at millimeter-wave frequencies, the dielectric waveguide 100 may also comprise a cladding 130 disposed around the inner core 1 10 for further confining the propagating electromagnetic field to the inner core 1 10. As shown in Fig. 1 , the cladding 130 may be provided directly adjacent to and surrounding the dielectric medium 120 so as to reinforce the effect of wave confinement provided by the high dielectric, low loss inner core 1 10. The cladding 130 may be provided as a single layer of dielectric material, as illustrated in Fig. 1 . However, the cladding 130 could also be implemented as a multilayered structure comprising a plurality of dielectric layers with respective dielectric constants that decrease outwards from the center of the dielectric waveguide 100. The cladding 130 is preferably made of a non-metallic material(s) so as to avoid the use of metals in the composition of the dielectric waveguide 100. In particular, the cladding 130 is preferably made of a dielectric material(s), such as polymer material, having a dielectric constant lower than the dielectric constant of the dielectric medium 120. The material(s) selected for the cladding 130 may also exhibit a low tangent loss (tan δ) with a value comparable to the tangent loss of the dielectric medium 120 so as to minimize the transmission loss introduced by the cladding 130 itself. However, since the portion of the electromagnetic field propagating outside the inner core 1 10 is already reduced due to the dielectric properties of the dielectric medium 120, the cladding 130 should not introduce significant transmission loss at millimeter-wave frequencies. For instance, very low attenuation (< 2 dB/m) may be achieved for a configuration in which the dielectric waveguide 100 has an inner core 1 10 of quartz and a cladding layer 130 made of polytetrafluoroethylene (PTFE), which exhibits a dielectric constant

8 _r and a loss tangent tan δ of about 2.1 and 0.00015, respectively, at 10 GHz. Another suitable material for the cladding layer 130 is a combination of PTFE and polyethylene (PE).

Simulation results of the signal attenuation for a dielectric waveguide 100 designed for a carrier frequency of 140 GHz having an inner core 1 10 of quartz (2 x 1 .5 mm ² ) and a circular cladding layer 130 based on PTFE (having an outer diameter of d = 4mm), and using air as the surrounding medium, show that less than 3% of the propagating electromagnetic field is carried in the surrounding medium (air) without using metallic shields. In contrast thereto, simulation results obtained for conventional dielectric waveguide fibers having a core made of low density polyethylene (LPDE) and a cladding of polypropylene (PP), which exhibit dielectric constants ε _Γ of about 2.4 and 2.15, respectively, show that a transmission loss of about 3.8 dB/m may be achieved at 140 GHz for an outer radius of the fiber of about 0.48 mm and a core with a rectangular cross-section of 0.4 mm x 0.2 mm. However, in this case the low transmission loss per meter is due to the significant part of the field components propagating in the air surrounding the fiber, leading to the transmitted wave being highly sensitive to touch and/or strongly influence by coupling with surrounding components.

In the configuration of the dielectric waveguide 100 shown in Fig. 1 , the dielectric medium 120 is provided as a solid core of high dielectric constant, low-loss dielectric material, such as alumina or quartz. However, due to the stiffness of these materials in their solid form, the dielectric waveguide 100 is rather stiff and difficult to bend.

Figs. 2 and 3 illustrate alternative configurations for a low-loss dielectric waveguide having improved flexibility in comparison to the dielectric waveguide 100 with the solid core 1 10 described above. Referring to Fig. 2, the dielectric waveguide 200 comprises an inner core 210, in which the dielectric medium 220 for carrying the electromagnetic field associated with the transmitted signal comprises a plurality of fibers 225 made of a high dielectric constant, low-loss dielectric material. Each single fiber 225 may be made of the same dielectric material(s) used for the solid dielectric medium 120 of the inner core 1 10 described above, such as quartz or alumina, so as to provide dielectric properties similar to those of the solid core 1 10. However, due to the lower cross-section of each single fiber 225, their flexibility is increased as compared to a solid core of a larger diameter, thereby providing a flexible dielectric medium 220. The fibers 225 are preferably arranged in the form of a bundle and provided in sufficient number such as to provide a dielectric medium 220 with a cross-section of about a few millimeters, which is suitable for most millimeter-wave signal applications. Due to the relatively large wavelength of millimeter- wave signals, the use of a bundle of fibers as dielectric medium 220, instead of the solid core 1 10, does not significantly influence the propagation of the wave signal along the dielectric waveguide 200.

As illustrated in Fig. 2, each single fiber 225 of the bundle may continuously extend over the full length L of the dielectric waveguide 200. In addition, the single fibers 225 may be arranged so as to run substantially in parallel to each other so as to provide a compact arrangement and avoid breaking of the single fiber filaments.

Broken filaments in fiber bundles is a critical issue for optical waveguides used in optical links for the reason that they cause the light to couple out of the optical waveguide. At millimeter- wave frequencies, filament discontinuities along the length of the fiber bundle are not critical due to the larger wavelength of the transmitted signal. Accordingly, an alternative configuration of the low-loss dielectric waveguide 200 may be realized in which the dielectric medium of the inner core comprises one or more bundles of shorter fibers made of the same dielectric material with high dielectric constant and low tangent loss described above, but in which the shorter fibers do not have the full length L of the dielectric waveguide. In order to improve the dielectric properties of the bundle of shorter fibers, these may be spun so as to form a yarn or thread, like in textile yarns. This makes possible to bundle shorter filaments to a flexible yarn of the required diameter and provide a flexible dielectric medium for the inner core of the dielectric waveguide 200. Although the inner core 210 of the dielectric waveguide 200 illustrated in Fig. 2 has a cross- section with a shape that is approximately rectangular, the fibers 225 or the short filaments forming the dielectric medium of the inner core may be arranged in a bundle having a cross- section of another shape, such as a square, circular or an elliptical shape. In addition, similarly to the waveguide configuration described with reference to Fig. 1 , the flexible core 210 may be further processed by applying a cladding 230 that surrounds the inner core 210. Preferably, the cladding 230 is arranged directly adjacent to the flexible core 210 and may be made of a non-metal material(s), similarly to the cladding 130 described above.

An alternative configuration for realizing a flexible low attenuation, low-loss dielectric waveguide 300 for transmission of millimeter-wave signals will now be described with reference to Fig. 3. In the illustrated configuration, the dielectric waveguide 300 comprises an inner core 310 in which the dielectric medium 320 for carrying the propagating wave is made of a powder and/or granulate of dielectric material of high dielectric constant and low loss tangent, i.e. with ε _Γ > 3.0 and tan 8 < 0.001 , that fills the volume of the inner core 310. For instance, the dielectric medium 320 may be a core of quartz powder or of alumina granulate. The effective dielectric constant and the loss tangent of the dielectric medium 320 may be lower than that of a solid core made of the same dielectric material due to a portion of the granular dielectric medium 320 being filled with another medium, such as air which exhibits 8 _r = 1 .0 and tan δ = 0.0.

Nevertheless, by using powders of dielectric material(s) characterized by a high dielectric constant, such as alumina or quartz, an effective dielectric constant 8 _r higher than 3.0 and low transmission loss might still be achieved. For instance, in the case of an inner core of alumina, a reduction of the effective dielectric constant ε _Γ from about 9.0 exhibited by a solid core to about

4.0 to 5.0 for the granulate core can be expected. Thus, it is possible to realize a dielectric waveguide having low-loss properties at millimeter-wave signal frequencies and having improved flexibility as compared to a dielectric waveguide with a core of the same solid dielectric material.

Similarly to the previous embodiments, the dielectric waveguide 300 may also comprise a cladding layer 330 surrounding the inner core 310 so as to further reduce the interaction of the propagating field with external agents. As described above, the cladding 330 may be directly adjacent to the inner core 310, and is preferably made of a non-metallic material, for e.g. polymer materials including PTFE, PE or other polymers known in the field.

A jacket (not shown) may be provided around the inner core and the cladding of the dielectric waveguides described above.

Consequently, due to the relatively large wavelength of millimeter wave signals, the wave propagation will not be significantly influenced by the fact of the carrier medium of the inner core being implemented as a bundle of single fibers having the total waveguide length, a yarn of shorter fiber filaments and/or as a powder/granulate as compared to a solid core of the same high dielectric, low loss dielectric material. In addition, the enhanced flexibility of a core made of a powder material and/or of bundles of fibers with a lower diameter than a core of pure, solid material makes possible to significantly improve the bending properties of the dielectric waveguide without compromising the low loss characteristics associated with the high dielectric, low loss dielectric material discussed above.

In addition, one or more dielectric waveguides according to any of the configurations described above may be integrated in a single cable, thereby providing a low-loss cable for high speed communications.

LIST OF REFERENCE NUMERALS

100 dielectric waveguide

1 10 inner core

120 dielectric medium

130 cladding layer

200 dielectric waveguide

210 inner core

220 dielectric medium

225 plurality of fibers

230 cladding layer

300 dielectric waveguide

310 inner core

320 dielectric medium

330 cladding layer

L length of dielectric waveguide